The science of laser amplification is known and a multitude of scientific uses exists for such devices and in the industrial/commercial arena (optical fibre laser transmission for communications etc).
Use of lasers and in particular metal vapour and copper bromide lasers is also known, their use being equally diverse in the scientific industrial/commercial and medical fields.
Copper bromide lasers are additionally known for their very high optical brightness gain (approx 10,000).
One application that appears particularly suitable for the use of a laser occurs in the relatively new field of Laser Direct Imaging (LDI).
Forming an image on a photo sensitive film referred to generally as photo-lithography. It is commonly employed to expose patterns via a mask on to devices such as integrated circuits, integrated circuit masks, flat panel displays and printed circuit boards as well as application in the printing industry.
A conventional photolithographic process begins with the coating of a work piece with a layer of photo-resist. Selected regions of the photo-resist are acted upon by light shone (typically Ultra Violet (UV)) through the prepared film or mask inducing a chemical reaction in the photoresist. Either the illuminated regions or the non-illuminated regions (depending on the type of photo resist used) are then removed by a chemical process to leave a patterned layer covering the portion of the surface of the device that is to remain after further chemical processing to remove the uncovered layer portion.
The device is then subjected to a chemical process, such as etching to remove the uncovered portions of the upper layer of the work piece.
The photo chemical resistive layer is then cleaned off the device and tracks, pads, via hole surrounds and the like remain on the surface of the device.
A laser scanner is another device that can be used to expose light on to a surface. A scanner can position with great precision one or more focused and intensity modulated laser beams as a series of scan lines over an area of a work piece being patterned. That precision depends on the sharpness of the focus of the laser beam, the accuracy of modulation of the laser beam, the precision with which the laser beam moves across the layer being patterned and the synchronisation between the modulation and movement of the laser beam. The power of the laser will affect the photoresist accordingly. Essentially prior arrangements are a pixel by pixel serial scanning arrangement.
FIG. 1 shows an example of a prior art arrangement of a Laser Direct Imaging (LDI) system used for illuminating a printed circuit board. In this prior art example, an ultra-violet (UV) argon-ion laser system is coupled into an Acousto-Optical Modulator (AOM) where the continuous wave laser beam is in effect switched on and off (time modulated) in a manner determined by rasterized data supplied from a computer. The now modulated laser light passes through various lenses and is directed via mirrors, to a scanning arrangement. In the prior art example, scanning motion is realised through the use of a rotating “polygon”. The polygon is a multi-faced rotating mirror assembly and its speed of rotation together with the available laser power determines the energy available for photochemical changes to affect the photoresist coating on the surface of the work piece. After each linear scan, the board moves to the next row of a predetermined array of linear scans.
The faster the photo sensitive resist reacts to illumination, the faster the polygon can rotate and the shorter will be the time it takes to illuminate a line and thus a work piece. Thus the sensitivity of reaction of the resists, the polygon and AOM speed and laser output power become major factors in determining the productivity of prior art laser direct imaging systems using available photo sensitive resists.
It is possible to split the beam into multiple paths so as to expose the board along multiple lines or locations to further increase productivity.
The various optical systems used to deflect and manipulate the laser beam from its original output path through the system include reflective and refractive elements.
At present it seems that no significant improvement in laser powers at wave lengths below 532 nm can be seen in the near future thus most effort is being put into developing high sensitivity, dry UV resists having 10 mJ/cm2 or greater sensitivity. However, even this figure is only achieved in special formulations that are expensive, have short shelf life and a low contrast ratio. Exposure times become critical but increasing exposure periods will lead to lower productivity.
All this must occur with the highest possible work piece throughput and minimal material wastage and defects. Finding a suitable combination of laser frequency, power, resists and working rate that matches all of the above characteristics is the challenge at hand for LDI systems designers.
As will therefore become apparent, the LDI area is ripe for LDI apparatus that can provide a large number (many thousands) of pixels in a parallel rather than a serial format. An amplification system as proposed herein will assist in this regard. However, it should be noted that the LDI application described in detail herein is an example only, and other applications for image amplification are not excluded from the scope of the invention generally disclosed herein.